Singletons and Inheritance

Question

Hello all,
I want to write a set of classes in the following way. Say I have a class called Base from which SubX, SubY, and SubZ are derived. I would like Base to be a singleton class, with a getInstance() method, so that one and only one of the subclasses can ever be instantiated. Which particular subclass to instantiate is determined at run time.
// Create (or load or initialize...) subclass dynamically at run time (let's say SubY)
// From now on, I cannot instantiate any more classes derived from Base in this VM (i.e. no instances of SubX, SubY, or SubZ).
// Base.getInstance() now always returns the SubY singleton.
Anyone have ideas on how to do this?
Thanks in advance...
Matt

Answer 1

I think the main problem here is that to do this within the hierarchy is that it means a base class must have knowledge of its derrived classes. This is a pretty big OO no no (one reason for this is that it makes extending your hierarchy pretty difficult).
I think that what you want is a third party who manages your constraint (i.e, only one translator should be used, and should be available to everyone).
Ive got a couple of solutions - the first one would leave your translators not being singletons, the other one would.......
Solution one:
Have a singleton TranslatorFactory who provides you with a single translator:
Heres some classes/interfaces we need to know about:
public interface Translator
  // Your translation stuff:
  public void doTranslate();
public class ConcreteTranslatorA
  public ConcreteTranslatorA()
    // Allow construction....
  public void doTranslate()
    // Some Translation stuff
// The manager - exceptions dealt with in terms of Exception in this example just for clarity:
public class TranslatorFactory
  private TranslatorFactory mInstance;
  private Translator mTranslator;
  private static final String TRANSLATOR_PROPERTY="TRANSLATOR";
  public static TranslatorFactory GetInstance() throws Exception
    // usuall singleton stuff - create/provide instance
  public TranslatorFactory throws Exception
    // Create the translator here. You might do this by getting the class name from system properties, or by any other method you like  
   String aClassName=System.getProperty(TRANSLATOR_PROPERTY);
    mTranslator=(Translator)(Class.forName(aClassName).newInstance());
  public Translator getTranslator()
    return mTranslator;
}If you really want to prevent your translator instances being instantiated, you can do this another way:
1) Make their constructors private.
2) Make the TranslatorFactory a registry of Translators. Each translator uses a static initialiser to make itself known to the registry (in terms of a Translator). The registry caches each instance against its class name. The factory can then configure which translator to use based on some property. This is a good approach to take if the property can change: I.e, different translators can be provided at different times without incurring extra creation.
public class TranslatorRegistry
  private Map mTranslatorMap;
  public TranslatorRegistry GetInstance()
    // usual singleton stuff....
  public void register(Translator aTranslator)
    // cache the instance against its class name in the mTranslatorMap.
  public Translator getTranslator()
    // decide which translator to use from the cache, and supply it.
public class ConcreteRegisteringTranslatorA
  static
    // perform automatic registration:
    TranslatorRegsitry.GetInstance().register(new ConcreteRegisteringTranslatorA());
  protected ConcreteRegisteringTranslatorA
    // cant be constructed externally, but allow derrivation by keeping constructor protected
  public void doTranslation()
    // translation
}